This invention relates to a mass spectrometer (MS) which uses the Fourier transform ion cyclotron resonance (FTICR) technique to determine the mass of ions and more particularly to a method for forming the sealing seat in the sampling valve in the FTICR MS which is used to admit sample to the high vacuum environment of the FTICR MS.
When a gas phase ion at low pressure is subjected to a uniform static magnetic field, the resulting behavior of the ion is determined by the magnitude and orientation of the ion velocity with respect to the magnetic field. If the ion is at rest, or if the ion has only a velocity parallel to the applied field, the ion experiences no interaction with the field.
If there is a component of the ion velocity that is perpendicular to the applied field, the ion will experience a force that is perpendicular to both the velocity component and the applied field. This force results in a circular ion trajectory that is referred to as ion cyclotron motion. In the absence of any other forces on the ion, the angular frequency of this motion is a simple function of the ion charge, the ion mass, and the magnetic field strength:
xcfx89=qB/mxe2x80x83xe2x80x83Eq. 1
where: xcfx89=angular frequency (radians/second)
q=ion charge (coulombs)
B=magnetic field strength (tesla)
M=ion mass (kilograms)
The FTICR MS exploits the fundamental relationship described in Equation 1 to determine the mass of ions by inducing large amplitude cyclotron motion and then determining the frequency of the motion. The first use of the Fourier transform in an ion cyclotron resonance mass spectrometer is described in U.S. Pat. No. 3,937,955 entitled xe2x80x9cFourier Transform Ion Cyclotron Resonance Spectroscopy Method And Apparatusxe2x80x9d issued to M. B. Comisarow and A. G. Marshall on Feb. 10, 1976.
The ions to be analyzed are first introduced to the magnetic field with minimal perpendicular (radial) velocity and dispersion. The cyclotron motion induced by the magnetic field effects radial confinement of the ions; however, ion movement parallel to the axis of the field must be constrained by a pair of xe2x80x9ctrappingxe2x80x9d electrodes. These electrodes typically consist of a pair of parallel-plates oriented perpendicular to the magnetic axis and disposed on opposite ends of the axial dimension of initial ion population. These trapping electrodes are maintained at a potential that is of the same sign as the charge of the ions and of sufficient magnitude to effect axial confinement of the ions between the electrode pair.
The trapped ions are then exposed to an electric field that is perpendicular to the magnetic field and oscillates at the cyclotron frequency of the ions to be analyzed. Such a field is typically created by applying appropriate differential potentials to a second pair of parallel-plate xe2x80x9cexcitexe2x80x9d electrodes oriented parallel to the magnetic axis and disposed on opposing sides of the radial dimension of the initial ion population.
If ions of more than one mass are to be analyzed, the frequency of the oscillating field may be swept over an appropriate range, or be comprised of an appropriate mix of individual frequency components. When the frequency of the oscillating field matches the cyclotron frequency for a given ion mass, all of the ions of that mass will experience resonant acceleration by the electric field and the radius of their cyclotron motion will increase.
An important feature of this resonant acceleration is that the initial radial dispersion of the ions is essentially unchanged. The excited ions will remain grouped together on the circumference of the new cyclotron orbit, and to the extent that the dispersion is small relative to the new cyclotron radius, their motion will be mutually in phase or coherent. If the initial ion population consisted of ions of more than one mass, the acceleration process will result in a multiple isomass ion bundles, each orbiting at its respective cyclotron frequency.
The acceleration is continued until the radius of the cyclotron orbit brings the ions near enough to one or more detection electrodes to result in a detectable image charge being induced on the electrodes. Typically these xe2x80x9cdetectxe2x80x9d electrodes will consist of a third pair of parallel-plate electrodes disposed on opposing sides of the radial dimension of the initial ion population and oriented perpendicular to both the excite and trap electrodes. Thus the three pairs of parallel-plate electrodes employed for ion trapping, excitation, and detection are mutually perpendicular and together form a closed box-like structure referred to as a trapped ion cell. FIG. 1 shows a simplified diagram for a trapped ion cell 12 having trap electrodes 12a and 12b; excite electrodes 12c and 12d; and detect electrodes 12e and 12f. 
As the coherent cyclotron motion within the cell causes each isomass bundle of ions to alternately approach and recede from a detection electrode 12e, 12f, the image charge on the detection electrode correspondingly increases and decreases. If the detection electrodes 12e, 12f are made part of an external amplifier circuit (not shown), the alternating image charge will result in a sinusoidal current flow in the external circuit. The amplitude of the current is proportional to the total charge of the orbiting ion bundle and is thus indicative of the number of ions present. This current is amplified and digitized, and the frequency data is extracted by means of the Fourier transform. Finally, the resulting frequency spectrum is converted to a mass spectrum using the relationship in Equation 1.
Referring now to FIG. 2, there is shown a general implementation of a FTICR MS 10. The FTICR MS 10 consists of seven major subsystems necessary to perform the analytical sequence described above. The trapped ion cell 12 is contained within a vacuum system 14 comprised of a chamber 14a evacuated by an appropriate pumping device 14b. The chamber is situated within a magnet structure 16 that imposes a homogeneous static magnetic field over the dimension of the trapped ion cell 12. While magnet structure 16 is shown in FIG. 2 as a permanent magnet, a superconducting magnet may also be used to provide the magnetic field.
Pumping device 14b may be an ion pump which is an integral part of the vacuum chamber 14a. Such an ion pump then uses the same magnetic field from magnet structure 16 as is used by the trapped ion cell 12. An advantage of using an integral ion pump for pumping device 14b is that the integral ion pump eliminates the need for vacuum flanges that add significantly to the volume of gas that must be pumped and to the weight and cost of the FTICR MS. One example of a mass spectrometer having an integral ion pump is described in U.S. Pat. No. 5,313,061.
The sample to be analyzed is admitted to the vacuum chamber 14a by a sample introduction system 18 that may, for example, consist of a leak valve or gas chromatograph column. The sample molecules are converted to charged species within the trapped ion cell 12 by means of an ionizer 20 which typically consists of a gated electron beam passing through the cell 12, but may consist of a photon source or other means of ionization. Alternatively, the sample molecules may be created external to the vacuum chamber 14a by any one of many different techniques, and then injected along the magnetic field axis into the chamber 14a and trapped ion cell 12.
The various electronic circuits necessary to effect the trapped ion cell events described above are contained within an electronics package 22 which is controlled by a computer based data system 24. This data system 24 is also employed to perform reduction, manipulation, display, and communication of the acquired signal data.
The FTICR MS 10 needs an ultra high vacuum to operate. The sensitivity of the FTICR MS 10 is such that an extremely small amount of the sample is required for a complete analysis. As was described above, the sample introduction system may consist of a leak valve. One example of such a valve is disclosed in U.S. Pat. No. 3,895,231 (xe2x80x9cthe ""231 Patentxe2x80x9d) which issued on Jul. 15, 1975. The valve of the ""231 Patent includes a diamond tipped or steel needle that closes the gas flow path. Another example of such a valve is shown in U.S. Pat. No. 4,560,871. The valve shown therein has a ball that closes the gas flow path.
As is well known to those in the art, the higher the temperature that a MS operates at the greater the range of the MS as the higher operating temperature gives rise to a greater range of gas phase molecules that can be analyzed in the MS. The operating temperature of the MS is limited by the materials used in the gas flow path.
Further the FTICR MS is a pulsed device and thus requires a sample volume at least 1000 times smaller than conventional mass spectrometers. High vacuum pumps required to reach the extremely low pressure needed by the FTICR MS can be very expensive. The cost is proportional to the volume and pressure of the sample gas to be pumped. Therefore, the FTICR MS requires a valve for operation at low leak rates that must have long life and virtually perfect seals. The present invention allows such a seal to be made such that the valve has undetectable leak rates and lifetimes in excess of one billion cycles of operation.
The present invention is a method for forming a seat in a pulsed sampling valve. The method includes the step of providing a seat support structure that has a passageway therein for admitting gas sample into a mass spectrometer vacuum chamber when the valve is opened. The method also includes the step of drilling a hole in the support structure. The drilled hole communicates with the passageway. The method further includes the step of aligning a ball with the support structure passageway central axis. The method further also includes the step of exerting a force to bring the ball in contact with the edges of the hole. The force initially exceeds the yield strength of the material from which the edges are formed. The edges deform in response to the force exerted through the ball until the area of contact between the edges and the ball increases to stop the deformation.
The present invention is a mass spectrometer that includes a vacuum chamber and a pulsed sampling valve that has a passageway for allowing gas sample to enter the vacuum chamber when the valve is opened. The valve has a seat formed by a method that includes the step of providing a seat support structure that has the passageway. The method further includes the step of drilling a hole in the support structure. The drilled hole communicates with the passageway. The method also includes the step of aligning a ball with the support structure passageway central axis. The method also further includes the step of exerting a force to bring the ball in contact with the edges of the hole. The force initially exceeds the yield strength of the material from which the edges are formed. The edges deform in response to the force exerted through the ball until the area of contact between the edges increases to stop the deformation.
The present is a method for forming a seat in a pulsed sampling valve. The method has the step of providing a precision seat comprising a ceramic ball and a ceramic seat support structure having a ceramic seat where the ball meets said support structure. The structure has a hole which is closed when the ball meets the support structure and a passageway therein. The hole communicates with the passageway. The passageway is for admitting gas sample into a mass spectrometer vacuum chamber when the valve is opened. The method also includes the step of coating the ceramic seat with one or more layers of a metal that will yield when a force is applied to the ball. The method further includes the step of exerting a force to bring the ball in contact with the edges of the metal coated ceramic seat. The force initially exceeds the yield strength of the metal. The metal coated seat edges deforms in response to the force exerted through the ball until the area of contact between the metal and the ball increases to stop the deformation.